Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ASSETS
Investment in subsidiaries	$ 448,996	$ 383,717
Other assets	443,165	474,364
Total assets	13,471,813	12,478,704	$ 11,031,896
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	135,000	135,000
Long-term debt	302,323	298,025
Other liabilities	453,126	384,173
Total liabilities	11,681,734	10,911,419
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0	0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2020, 66,316,797; 2019, 66,088,808	66	66
Additional paid-in capital	488,367	480,962
Retained earnings	1,434,634	1,352,539
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturity securities	366,285	230,448
Net funded status of benefit plans	(11,165)	(10,767)
Treasury stock, at cost, 2020, 24,902,579 shares; 2019, 24,850,484 shares	(488,108)	(485,963)
Total shareholders' equity	1,790,079	1,567,285	$ 1,290,550
Total liabilities and shareholders' equity	13,471,813	12,478,704
Parent company
ASSETS
Investments and cash	4,146	1,453
Investment in subsidiaries	2,120,982	1,901,725
Other assets	58,852	62,442
Total assets	2,183,980	1,965,620
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	135,000	135,000
Long-term debt	248,323	248,025
Other liabilities	10,578	15,310
Total liabilities	393,901	398,335
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0	0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2020, 66,316,797; 2019, 66,088,808	66	66
Additional paid-in capital	488,367	480,962
Retained earnings	1,434,634	1,352,539
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturity securities	366,285	230,448
Net funded status of benefit plans	(11,165)	(10,767)
Treasury stock, at cost, 2020, 24,902,579 shares; 2019, 24,850,484 shares	(488,108)	(485,963)
Total shareholders' equity	1,790,079	1,567,285
Total liabilities and shareholders' equity	$ 2,183,980	$ 1,965,620